Recently, the image forming device such as the laser beam printer or the projector often employs the multibeam scanning device which is provided with a plurality of light sources arranged in a predetermined direction. In the image forming device employing the multibeam scanning device, the laser beams emitted by the plurality of light source are ON/OFF modulated in accordance with a modulation signal generated in accordance with image formation data, and the modulated laser beams are deflected by a deflector to scan on a surface to be scanned so that the laser beams scan thereon in a first direction. At the same time, by moving the incident position of the laser beam (i.e., a spot of formed by the laser beam incident thereon) on the surface to be scanned in a second direction, which is perpendicular to the scanning direction (first direction), copying or imaging of a two-dimensional image can be performed.
In the following description, the first direction is referred to as a main scanning direction, and the second direction is referred to as an auxiliary scanning direction. Further, when optical paths of the multibeam scanning device are developed, a plane including a central axis of a scanning optical system (i.e., an optical axis of the scanning optical system) and the main scanning direction will be referred to as a main scanning plane, and a plane including the central axis of the scanning optical system and the auxiliary scanning direction will be referred to as an auxiliary scanning plane.
Among the multibeam scanning devices described above, there are ones provided with an adjusting mechanism for adjusting the positions of the beam spots such that the beam spots formed on the surface to be scanned are spaced evenly, as disclosed, for example, in Japanese Patent Provisional Publication No. 2004-333862A.
In the above described publication, on a downstream side of each light source, a wedge prism which is rotatable about an axis parallel to an optical path of the laser beam is provided. By rotating the wedge prism, a condition where the light source is hypothetically displaced from an actual position, i.e., a condition where a virtual image of the light source is displaced, is created. With this configuration, the designer tries to change the position of each beam spot on the surface to be scanned by changing an incident angle of each laser beam with respect to the deflector (i.e., a deflecting surface).
With the configuration disclosed in the above described publication, however, the positions where the laser beams are incident on the deflector are changed. Such a change may cause vignetting and/or deterioration of performance in deflection. According to the above described publication, only a one-dimensional scanning is considered in the main scanning direction, and on the auxiliary scanning plane, the beams are converged on the deflection surface. Therefore, the above described publication does not deal with the above problem.
When both the main scanning and auxiliary scanning are to be performed in the image forming device on which the multibeam scanning optical system is mounted, if the laser beams emitted by the respective light sources are converged on the deflection surface, the deflection surface and the surface to be scanned become conjugate with each other. In such a case, regardless of the incident angle of the laser beam, an image height of the laser beam on the surface to be scanned remains constant. Such an optical system does not work as a scanning system. Therefore, in devices requiring that the beams should not be converged on the deflection surface, a change of the incident angle with respect to the deflection surface causes a change of the incident position thereon relatively largely in comparison with a device such as the above described publication which is configured that the beams are converged on the deflection surface on the auxiliary scanning plane. Therefore, in devices in which the beams cannot be converged on the deflection surface, effects of shift of the incident positions of the laser beams on the deflection surface cannot be overlooked, different from the disclosure of the above described publication.
Thus, in the devices which do not converge light beams on the deflection surface (e.g., a multibeam scanning device), if the positions of the beam spots are to be adjusted employing the configuration of the above described publication, the displacement of the incident positions of the beam spots as described above should be prevented effectively. Specifically, an adjusting mechanism that moves the light source to adjust the incident positions may be required, or the size of a large deflector, which can deal with a relatively large shift of the incident positions, should be increased.